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              <h1>Volatile指令重排语义 - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
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  {
    "question": "根据文档，Java中的`volatile`关键字主要提供了哪两个核心保证？",
    "options": [
      "原子性和互斥性",
      "可见性和禁止指令重排",
      "可见性和原子性",
      "线程安全和性能优化"
    ],
    "correctAnswerIndex": 1,
    "explanation": "`volatile`关键字的两大核心作用是：1. 保证可见性，即当一个线程修改了volatile变量的值，这个新值对其他线程是立即可见的。2. 禁止指令重排，通过内存屏障确保volatile变量相关的操作不会被编译器或处理器重排序，从而保证多线程环境下的程序执行顺序。",
    "source": "文档中「1. volatile关键字的基本作用」部分",
    "hint": "文档开头就总结了该关键字的两个基本作用，它们是解决多线程并发问题的关键。"
  },
  {
    "question": "`volatile`关键字是如何实现“禁止指令重排”这一特性的？",
    "options": [
      "通过为变量的每次访问加锁，类似于`synchronized`。",
      "通过在编译时将代码标记为不可优化，禁止所有编译器优化。",
      "通过强制所有线程在访问该变量后进入休眠状态。",
      "通过在变量的读写操作前后插入内存屏障 (memory barrier)。"
    ],
    "correctAnswerIndex": 3,
    "explanation": "JVM为了实现`volatile`的禁止重排语义，会在对`volatile`变量进行读写操作的前后插入特定的CPU指令——内存屏障。内存屏障可以确保其前后的指令不会被重排序，从而保证了代码的执行顺序符合预期。",
    "source": "文档中「3. volatile与指令重排」和「5. 内存屏障」部分",
    "hint": "文档中提到，JVM会插入一种特殊的CPU指令来保证操作顺序的正确性。"
  },
  {
    "question": "与`synchronized`关键字相比，`volatile`关键字缺少了哪项关键的并发保证？",
    "options": [
      "可见性",
      "禁止指令重排",
      "原子性和互斥性",
      "线程间的通信能力"
    ],
    "correctAnswerIndex": 2,
    "explanation": "`synchronized`关键字可以保证一个代码块的原子性（操作不可分割）和互斥性（同一时间只有一个线程能进入）。而`volatile`仅保证变量的可见性和禁止指令重排，它不保证像 `i++` 这样的复合操作的原子性。",
    "source": "文档中「6. volatile与其他同步机制的比较」部分",
    "hint": "思考一下，“锁”提供了什么`volatile`所不具备的保护能力？"
  },
  {
    "question": "在文档提供的代码示例中，如果将`flag`变量的`volatile`关键字移除，最可能发生什么问题？",
    "options": [
      "程序将无法编译通过。",
      "`thread2`可能在`flag`为`true`时读取到`count`的旧值`0`，因为赋值操作可能被重排。",
      "`thread1`和`thread2`会发生死锁。",
      "`thread2`将永远无法看到`flag`变为`true`。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "如果没有`volatile`，编译器或CPU可能会为了优化而将`flag = true`指令重排到`count = 1`之前。这样，`thread2`可能会先看到`flag`变为`true`并执行打印语句，但此时`count = 1`的赋值操作还未执行，导致打印出陈旧的值`0`。`volatile`通过禁止重排保证了`count=1`在`flag=true`之前对`thread2`可见。",
    "source": "文档中「4. 示例与解释」部分",
    "hint": "文档中解释该示例时，重点强调了移除`volatile`后两条赋值语句的执行顺序问题。"
  },
  {
    "question": "当一个线程写入一个`volatile`变量时，Java内存模型（JMM）会确保发生什么？",
    "options": [
      "通知所有其他线程暂停执行，直到写操作完成。",
      "仅更新当前线程的本地缓存，等待稍后同步到主内存。",
      "立即将该变量的新值刷新到主内存中，并使其对其他线程可见。",
      "清除所有其他线程的CPU缓存。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "对`volatile`变量的写操作具有特殊的内存语义。JVM会确保该操作立即将变量的值从工作内存（如CPU缓存）写入主内存。这个写操作会使其他线程中缓存的该变量的值失效，从而迫使它们在下次读取时必须从主内存重新获取最新值，保证了可见性。",
    "source": "文档中「3. volatile与指令重排」部分的“写入volatile变量”小节",
    "hint": "思考一下`volatile`是如何实现“可见性”的。一个线程修改后，如何让其他线程“看到”？"
  },
  {
    "question": "根据文档，什么是“指令重排”？",
    "options": [
      "是程序员为了代码可读性而手动调整代码行的顺序。",
      "是多线程环境下为了避免死锁而自动调整加锁顺序的机制。",
      "是编译器和处理器为了提升性能，在不改变单线程程序最终结果的前提下，改变代码指令执行顺序的一种优化手段。",
      "是操作系统在调度线程时，随机改变线程执行先后顺序的行为。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "指令重排是编译器和处理器层面的一种性能优化技术。它会分析指令间的依赖关系，并在不影响单线程程序正确性的情况下，重新安排指令的执行顺序，以更充分地利用CPU的执行单元。但在多线程环境中，这种重排可能破坏程序员预期的同步逻辑。",
    "source": "文档中「2. 指令重排的概念」部分",
    "hint": "文档中对这个概念有专门的定义，说明了它是谁（编译器和处理器）为了什么（性能）而做的。"
  },
  {
    "question": "以下哪项是文档中提到的`volatile`关键字的典型应用场景？",
    "options": [
      "保护需要互斥访问的复杂数据结构，如`HashMap`。",
      "用于对一个共享计数器进行原子性的自增操作（如 `counter++`）。",
      "作为线程间的状态标志变量，例如用于安全地停止一个线程。",
      "实现一个高性能的、可重入的锁。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "文档明确指出，`volatile`非常适合用作简单的状态标志。例如，一个线程可以通过检查一个`volatile boolean`标志来决定是否继续执行循环，而另一个线程可以改变这个标志的值来安全地通知第一个线程停止。这完美利用了`volatile`的可见性保证，且开销比锁更低。",
    "source": "文档中「7. 实际应用」部分",
    "hint": "文档最后一部分列举了`volatile`的常见用途。考虑一下哪个选项最符合一个“简单状态”的描述。"
  }
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            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  